add more to `nix flake check`

`nix flake check` currently:
https://github.com/haskell/hackage-server/actions/runs/5983383840/job/16233463312

```
warning: ignoring untrusted flake configuration setting 'allow-import-from-derivation'.
Pass '--accept-flake-config' to trust it
warning: ignoring untrusted flake configuration setting 'extra-substituters'.
Pass '--accept-flake-config' to trust it
warning: ignoring untrusted flake configuration setting 'extra-trusted-public-keys'.
Pass '--accept-flake-config' to trust it
evaluating flake...
warning: unknown flake output 'haskellFlakeProjectModules'
warning: 'https://hydra.iohk.io/' does not appear to be a binary cache
copying path '/nix/store/7nilm17fcdna4zp6jcx7zgyf4mi0swl4-all-cabal-hashes-component-attoparsec-aeson-2.1.0.0' from 'https://hackage-server.cachix.org/'...
building '/nix/store/cldccmxpfcrbmg744w49xs8qsfr7ciq0-cabal2nix-attoparsec-aeson-2.1.0.0.drv'...
warning: The check omitted these incompatible systems: aarch64-darwin, aarch64-linux, x86_64-darwin
Use '--all-systems' to check all.
```

It also checks that `nix flake develop` can start successfully on Mac and Linux.

More Hackage tests could be included in `nix flake check`.

By default, all tests are included in `nix build`. Those tests can be disabled here: https://github.com/haskell/hackage-server/blob/613f5eb2f24d6ccbdef349d65f38247425ab64df/flake.nix#L21-L26